1. Field of the Invention
The invention relates to a wheel rolling bearing device.
2. Description of Related Art
In a wheel rolling bearing device (also referred to as “wheel hub unit”), an inner-side inner ring raceway surface and an outer-side inner ring raceway surface are formed at a predetermined axial interval on the outer peripheral surface of a hub spindle of a hub wheel to which a wheel is fitted. An inner-side outer ring raceway surface and an outer-side outer ring raceway surface are formed at a predetermined axial interval on the inner peripheral surface of an outer ring member arranged on the outer periphery of the hub spindle. Inner-row balls are rollably arranged between the inner-side inner ring raceway surface of the hub spindle and the inner-side outer ring raceway surface of the outer ring member, and outer-row balls are rollably arranged between the outer-side inner ring raceway surface of the hub spindle and the outer-side outer ring raceway surface of the outer ring member. In the thus configured wheel rolling bearing device, a pitch diameter of the inner-row ball set and a pitch diameter of the outer-row ball set each are increased to increase the number of the inner-row balls and the number of the outer-row balls to thereby make it possible to achieve high stiffness. However, if the pitch diameter of the inner-row ball set and the pitch diameter of the outer-row ball set each are increased, the size of the outer ring member also increases. As a result, the weight of the outer ring member increases. In addition, if the size of the outer ring member increases, the outside diameter of a fitting shaft portion of the outer ring member may be excessively large with respect to an assembling hole of a vehicle body-side member, such as a knuckle and a carrier, of a suspension of a vehicle. As a result, it may be impossible to fit the fitting shaft portion into the assembling hole. For this reason, there is known the following technique. According to the technique, in order to achieve weight reduction and high stiffness while maintaining the outside diameter of the fitting shaft portion of the outer ring member at such a diameter that the fitting shaft portion is able to be fitted into the assembling hole of the vehicle body-side member, the pitch diameter of the outer-row ball set is set to be larger than the pitch diameter of the inner-row ball set. Thus, the number of the outer-row balls is larger than the number of the inner-row balls (for example, see Japanese Patent Application Publication No. 2004-108449 (JP-A-2004-108449)).
If the pitch diameter of the outer-row ball set is set to be larger than the pitch diameter of the inner-row ball set to make the number of the outer-row balls larger than the number of the inner-row balls, weight reduction and stiffness increase are promoted effectively. However, if the pitch diameter of the outer-row ball set is increased, the outer-row balls tend to easily run on the outer-side raceway shoulder when a load during rotation of the bearing is applied to the outer-row balls. When the outer-row balls run on the outer-side raceway shoulder, edge stress occurs at a boundary edge portion between the outer-side outer ring raceway surface and the outer-side raceway shoulder. As a result, the bearing service life may be shortened.